Amino-grafted Cu and Sc Metal-Organic Frameworks involved in the green synthesis of 2-amino-4H-chromenes. Mechanistic understanding.

Abstract

In this work, we report for the first time a new methodology for the eco-synthesis of 2-amino-4H-chromenes 1, from salycilaldehydes and cyano compounds, under solvent-free and mild conditions, using amino-grafted MOFs as catalysts. The selected MOFs − commercial CuBTC and MIL-100(Sc) previously synthetized in our laboratories − can be easily functionalyzed with amines of different nature showing notable differences in their composition and textural properties. The total or partial functionalization of the metal centers in starting MOFs is strongly depending on the functionalization method used. Our results indicate that the catalytic performance is mainly conditioned by the type and concentration of basic sites, porosity of the samples barely showing any influence. The methodology herein reported could be considered as an environmental friendly alternative to the selective chromene synthesis, which allows to achieve high yields in relatively short reaction times (up to 90% over 1 h), using notably small amounts of easily prepared catalysts. Furthermore, our experiments in combination with theoretical calculations strongly suggest that free-amine groups in ethylenediamine (EN) functionalized catalysts can act either as individual catalytic sites, as for ENM/ CuBTC sample, in which all metal centers are functionalized with EN ligands and shows the highest concentration of basic catalytic sites, or acting in cooperation with the closest metal centers in samples partially functionalized, as in the case of EN/CuBTC sample.